Description The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout (anadromous) usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species. The fish are often called salmon trout. Several other fish in the salmonid family are called trout, some are anadromous like salmon, whereas others are resident in freshwater only.

The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica. In some locations, such as Southern Europe, Australia and South America, they have negatively impacted upland native fish species, either by eating them, outcompeting them, transmitting contagious diseases, (like Whirling disease transmitted by Tubifex) or hybridization with closely related species and subspecies that are native to western North America.

Dimensions Up to 3'9" (1.1 m); 42 1/8 lbs (19.1 kg).

Endangered Status Sea-run populations of the Rainbow Trout, known as Steelheads, are on the U.S. Endangered Species List. Certain naturally spawned populations in California and Washington are classified as endangered, and other naturally spawned populations in California, Idaho, Oregon, and Washington are classified as threatened. The populations of these and other trout and salmon have been declining for years because of habitat alteration caused by logging, diversion of water from the streams and rivers the fish spawn in, development along those waterways, pollution, and the building of hydroelectric dams. Removal of trees by loggers and developers exposes the streams to sunlight, often warming the water to lethal temperatures. Erosion from logging roads and from cleared lands fills the water with sediment, which can also make it uninhabitable; and runoff from rooftops, roadways, and parking lots contains substances that can contaminate the water. Conservation efforts have been opposed by logging, agricultural, and development interests.